Generally speaking, white base glass and colorless transparent base glass is not counted as a color. The exception will be with Akro Agate 'Prize Name' and 'Special' corkscrews, where they included white base as a color within their packaged categories.
When describing how many colors are found in a marble, without a guideline, it becomes subjective, meaning you might see four and someone else may see six, when in reality, there were only two. Our guideline consensus is how many colors were used in production. This can be hard to determine, but keeping in mind the most common is two colors, reasonably common is three colors, and rarely did vintage machine-made manufacturing utilize four or more colors. Now the question is, is that really a fourth or fifth color, or blending? I refer you to the background image here. Though we may see five or six colors, it is a three-color marble with blends.
A brief study of primary colors and how to make secondary colors, as well as tinting with white, shading with black and tonal change with gray, will give you some insight on blending colors which can be observed in marble production. Some have argued that glass colors do not blend in the way paint does. I argue that at some level, without the aid of a microscope, we perceive the blending of glass colors in the same manner.
As different colors mingle in a crucible and through the stream, variable blending may occur. A red and yellow production may show orange, which is a blend of red and yellow. Maybe two, or more hue shifts of orange occur, making the marble appear to have four, or five different colors, though only two were used in production. Let's say the production also included blue. Now we can make green from the blue and yellow (though in glass it will often become a muddy gray), as well as making purple from the blue and red. Add varying degrees of hue shifts, and suddenly a three-color marble is being touted as a nine, ten or twelve color. Now, my keen eye with the aid of a loupe, will likely find thirty-seven different colors, but that doesn't make it a thirty-seven color marble, does it? The reality is, it's a 'Three color blended'. One more hitch is shadowing. Close observation in areas that appear to be black, if not created by very dark blends of complimentary colors such as red and green, orange and blue, or purple and yellow, which all combinations basically make mud, it may be observed, one color dipping, or diving down under another color creating a shadow, which at a glance, appears to be black. This is very commonly seen in corkscrews. That thin line of black, is more often a shadow.